This invention relates to diodes having lower series resistance in the forward-biased condition. The problem addressed herein is that diodes formed of silicon carbide have a much higher series resistance than diodes formed of other materials, such as silicon or gallium arsenide. In particular, a silicon carbide substrate adds significant resistance to the overall device and must be thinned before the diode is ready for use.
Diodes made of silicon carbide, however, are very attractive in radio frequency (“RF”) applications, such as high speed RF switches and limiters. The high breakdown strength and high thermal conductivity of silicon carbide enable much higher RF power levels and faster switching speeds than possible with silicon or gallium arsenide technologies. An important requirement in these applications is to obtain a low RF on-resistance in the forward biased condition to minimize RF losses.
For example, the on-resistance of PIN diodes mainly consists of two components—one due to the intrinsic layer (the “I-layer”) and the other due to the series resistance, Rs, of the substrate. The forward biased I-layer resistance is approximately proportional to the square of the thickness of this layer and is small in SiC devices due to the thin I-layer needed for a required breakdown voltage. The substrate series resistance, however, in silicon carbide diodes is much higher than that of silicon or gallium arsenide due to the higher substrate resistivity of silicon carbide.
Currently, the minimum achievable resistivity in SiC is 15 milliohms-cm (mΩ-cm), which cannot be reduced further without creating defects due to fundamental material properties. Due to this limitation, it is necessary to thin the SiC substrate to about 50-100 μm to achieve an acceptable on-resistance. Thinning the substrate, however, to such low values is undesirable due to the complicated processes involved and the associated yield loss.
The assignee of the invention herein has previously patented multiple techniques for forming diodes in semiconductor materials, such as U.S. Pat. No. 4,947,218 (Edmond 1990), the contents of which are incorporated herein by reference. Other entities have also patented diodes in silicon carbide, some focusing on the use of new materials in forming the contacts thereon. See, e.g., U.S. Pat. No. 6,544,674 (Tuller 2003).
In one attempt at minimizing the series resistance, U.S. Pat. No. 7,091,533 (Tihanyi 2006), discloses a Schottky diode with contacts on the same side of a semiconductor substrate. Tihani, however, fails to show any means by which the lateral current from anode to cathode can be controlled for optimal performance and offers no improvements in series resistance. Similarly, published U.S. Patent Application No. 2004/0097021 (Augusto) discloses photodiodes formed on active areas of standard CMOS devices. Augusto uses a common silicide layer to connect ohmic contacts between a PIN diode and other elements of the circuit. The Augusto patent application, however, does not disclose any relationship between the silicide layer and reducing the on resistance of the PIN diode. Neither of these prior publications directly addresses the presently claimed technique for reducing series resistance in devices formed of silicon carbide.
A need continues to exist in the art of semiconductor diodes, therefore, for reducing the on-resistance of silicon carbide devices under forward bias.